Encephalitozoon intestinalis is the second-most prevalent microsporidian in AIDS patients. SIV-infected rhesus macaques with CD4+CD29+ peripheral blood T cell levels above 10%, were inoculated orally or intravenously with E. intestinalis. IgG responses were detected by ELISA and Western blot approximately four-to-six weeks later and the strongest responses were expressed antigens with molecular weights of approximately 13, 32, 42, 50, 68, and 71 kDa. In addition, C57Bl mice given E. intestinalis orally generated IgG responses beginning 14 days after inoculation. The strongest responses detected by Western blot immunoassay were against antigens with molecular weights of approximately 13, 25, 32, and 50 kDa. Monospecific polyclonal antisera were raised against these antigens and are currently being assayed for complement fixation, opsonization, and neutralization activities.